1. Field of the Invention
The present invention relates generally to the field of radiation-curable silicone resins, and more particularly to such a resin formulated from an amine alcohol, tetra-alkoxysilane and a multifunctional acrylate.
2. Description of the Prior Art
A number of radiation-curable silicone resins are known in the prior art. Such resins are frequently used to provide abrasion-resistant coatings over easily-scratched substrates, such as polycarbonate, polyethylene and polyethylene terephthalate. The coated substrates are often used as a substitute for glass, as in the case of lenses for automobile head lamps.
Other uses may include application over plastic containers that are adapted for receiving beverages. In the case of non-refillable containers, the utility of the coating may be directed solely at providing a barrier film that decreases the gas permeability of the container. In the case of refillable containers, the utility of the coating may be directed at providing a barrier film that exhibits abrasion-resistant properties, either alone or in combination with barrier properties. Many silicone resins of this type can be impregnated with known slip aids to increase the lubricity of their cured surface.
Other applications for radiation-curable siloxane resins include casting articles of manufacture and use as bonding agents.
The silicone resin compositions of the prior art are generally formulated in a polar solvent, such as isopropyl alcohol, and utilize colloidal silica to impart abrasion resistance. The colloidal silica is usually added in the form of an aqueous dispersion.
Silicone resin compositions of the above type are disclosed in U.S. Pat. No. 4,455,205 to Olson et al. (issued Jun. 19, 1984 and assigned to General Electric Company, Schenectady, N.Y.) The coatings comprise an ultraviolet (UV) light-curable composition prepared from a multifunctional acrylate monomer, an acryloxy-functional silane and aqueous colloidal silica in a polar solvent. After mixing the components, the solvent and remaining water is removed in vacuo, with gentle heating, a step which is termed "stripping." A photoinitiator is then added so that the composition may be UV-cured.
Similarly, U.S. Pat. No. 4,486,504 to Chung, (issued Dec. 4, 1984 and also assigned to General Electric Company) discloses a UV-curable composition prepared from the addition of a multifunctional acrylate monomer to a mixture of acryloxy-functional silanes, and/or glycidoxy-functional silanes and aqueous colloidal silica. After stripping, a photoinitiator is added so that the composition may be UV-cured.
European Patent Application Publication No. 0 544 465 A1, assigned to General Electric Company, discloses a radiation-curable composition formed from: the hydrolysis product of an alkoxysilyl acrylate; aqueous colloidal silica; acrylate monomers; and a photoinitiator. Again, the example compositions are disclosed to be formulated in an alcohol solution.
U.S. Pat. No. 5,260,350 to Wright, discloses a radiation-curable composition which includes an aminoalkoxysilane and a multifunctional acrylate monomer (which form a Michael adduct) and colloidal silica in an aqueous dispersion. The composition is formulated in a polar solvent, such as isopropyl alcohol.
In the above prior art compositions, the water from the aqueous dispersion of colloidal silica causes the alkoxy groups on the silanes (and/or silyl acrylates) to hydrolyze.
The term "hydrolysis," in its strictest sense and as applied to alkoxysilanes, describes the reaction of the silanes, at the alkoxy sites, with water, thus forming silanols and alcohol. The silanols, however, are capable of undergoing a condensation reaction, to form Si--O--Si bonds and water. Accordingly, the term "hydrolysis," and its various other forms, as used herein, is not meant to include the subsequent condensation reaction. Hydrolysis, followed by condensation, will be so-described.
In the prior art compositions, it has been theorized that the colloidal silica includes residual silanol groups at the surface of the colloid particles. Further according to the theory, at least some of these residual silanol groups then undergo a condensation reaction with at least some of the silanol sites derived from the hydrolysis of the alkoxysilanes. Likewise, at least some of the hydrolyzed silanes condense with each other, thereby forming a plurality of loosely crosslinked resin nuclei, at least some of which are chemically bonded to particles of colloidal silica. According to the theory, upon curing, additional crosslinking provides a continuous resin network having colloidal silica particulate dispersed therethrough and chemically bonded therein. It is believed that the colloidal silica particles are largely responsible for imparting the compositions with abrasion-resistant properties.
In the prior art compositions, free radical polymerization of the remaining acrylate groups provides further crosslinking to the resin network, thereby "curing" the composition. The initiation of free radical polymerization can be carried out by the addition of known photoinitiators and exposure to (UV) radiation, or simple exposure to sufficiently high energy radiation such as an electron beam.
The use of aqueous colloidal silica in the formulations of such compositions has numerous drawbacks. Colloidal silica is known to have a tendency to gel, which tendency is particularly sensitive to changes in pH and solvent loss. For instance, with respect to the compositions disclosed in Wright, unless an acid is first added to reduce the residual alkalinity imparted by the aminosilane, the addition of colloidal silica to the Michael adduct/acrylate solution results in gelling or the precipitation of silica from colloidal suspension.
Furthermore, the amount of water added to the compositions of the prior art is dictated by the amount of water in the aqueous colloidal silica. Thus, in order to get an adequate amount of colloidal silica, the compositions are generally formulated with an excess of water. An excess of water creates several problems.
The degree of hydrolysis and condensation is dictated by the amount of water. Accordingly, the degree of cross-linking in the uncured composition cannot be readily controlled.
If excess water is left in the resin composition, it can have a deleterious effect on the finish of the cured coating, unless a lengthy drying time is employed before initiating cure. For that reason, the compositions are generally stripped of water. Stripping, however, also removes the polar solvent from the compositions in addition to water, greatly increasing the viscosity of the material. Therefore, polar solvent must generally be reintroduced in order to achieve a coating composition having a viscosity in a usable range.
The tendency of the prior art compositions to gel is greatly increased during the stripping step. Thus, the likelihood of manufacturing scrap materials is greatly increased.
In addition to the foregoing, aqueous colloidal silica represents a substantial portion of the cost associated with the production of the prior art resins.
Finally, the radiation-curable silicone resins of the prior art all utilize an expensive component which consists of a functionalized trialkoxysilane. The functional groups associated with the silanes permit copolymerization of the silanes with acrylate-functional components. Heretofore, such functional groups have always been attached to the trialkoxysilanes through an Si--C bond, the belief being that Si--O--C bonds are hydrolyzable and therefore cannot be effectively used to form a resin network capable of exhibiting substantial abrasion resistance.
For instance, the compositions of Chung and Olson et al. each utilize an acryloxy-functional alkoxysilane and the composition of Wright utilizes an amino-functional silane, each of which employs an Si--C bond.
Thus, a novel radiation-curable silicone resin material that did not require aqueous colloidal silica or an independently-prepared functionalized trialkoxysilane having an Si--C bond as a part of its formulation would be considered highly desirable.